Cavity resonator apparatus, including frequency control means



April 26, 1949. s. F. VARIAN 4 CAVITY RESONATOR APPARATUS, INCLUDING FREQUENCY CONTROL MEANS.

Filed Nov. 25, 1943 2 Sheet-Sheet 1 FIG. 3. 2 Wm INVENTOR SI URD F. VARIAN AT' 'ORNEY April 26, 1949. s. F.,VARIAN CAVITY RESONATOR APPARATUS, INCLUDING FREQUENCY CONTROL MEANS Filed Nov. 25, 1945 2 Sheets-Sheet 2 FIG. 4.

IO! '5 i I09 l 1 I02 5 I05 9s INVENTOR SIGURD F. ,VARIAN Mg/gm ATTORNk Y fatented Apr. 26, l94

CAVITY RESONATOR APPARATUS, INCLUD- ING FREQUENCY CONTROL MEANS Sigurd F. Varian,

ware

Garden City, N. Y., assignor to The Sperry Corporation,

a corporation of Dela- Application November 25, 1943, Serial No. 511,722

Claims.

This invention relates to frequency control of hollow resonator devices and is more particularly concerned with electrothermal arrangements and methods for controlling frequency in such devices.

Application Serial No. 580,528, filed March 2, 1945, is a division of the present application covering the disclosure of Figs. 4 and 5.

Electron discharge apparatus of the type wherein a beam of electrons is passed in energy exchanging relation with the electromagnetic field of a hollow resonator device are widely used, especially in ultra high frequency systems. In most of such systems, careful and reliable frequency control over the resonator circuit is important. It is known that the effective capacity and inductance of the circuit of a hollow resonator de vice depends mainly on the shape and/or volume of the resonator, and various mechanical and electrical arrangements and mechanisms have been heretofore suggested for variably controlling the circuit characteristics and frequency of the resonator by altering the resonator shape and/or Volume.

The present invention contemplates improvements in such electrical and electromechanical frequency control of hollow resonator devices.

The major object of the invention is to provide novel electrically energized, extremely sensitive, thermally responsive apparatus and methods of frequency control for hollow resonator devices.

A further object of the invention is to provide novel frequency control arrangements for a hollow resonator device wherein a thermally responsive frequency control member such as an expansible and contractible strut or wire operatively connected to said device is heated by electron bombardment.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims and the annexed drawings wherein:

Figure 1 is an elevation, partly in section, illustrating details of a preferred thermal tuning mechanism according to the invention;

Figure 1A is a fragmentary elevation view illustrating a modification of Figure 1;

Figure 2 is an elevation, partly in section, of a further embodiment of the invention wherein normally wasted electron energy is utilized for frequency control;

Figure 3 is an elevation, partly in section, of a still further embodiment of the phase of the invention illustrated in Figure 2;

Figure 4 is an elevation, partly in section, illustrating a further embodiment of the invention be located as close as possible including simultaneous and individual thermally actuated frequency control in a multiple hollow resonator device; and

Figure 5 is an elevation, partly in section, illustrating a further embodiment of the invention wherein a flexible non-rigid frequency control member is operatively connected to a hollow resonator device.

Referring now to Figure 1, an electron discharge device is contained within a cylindrical metal envelope l I mounted on a conventional type vacuum tube base 12 having prongs 13 for insertion in a supporting socket. Base l2 and envelope II are formed with mated lips which are welded or similarly joined as at M to complete a vacuum tight closure within the envelope.

A platform I5 within the envelope is supported by posts 16 arising from base l2, and similar support posts [1 extend upwardly from platform l5 for rigidly mounting a bridge plate l8 generally parallel to platform 15. Posts l1, platform l5, posts I1 and plate 18 provide a strong rigid frame solid with base l2 for mounting the various elements of the device within envelope H.

Platform I5 is formed with a surface recess 19 in which is seated a cylindrical hollow resonator 2| having relatively rigid side and bottom walls soldered or similarly conductively fastened to platform 15. Resonator 2| has a flexible upper wall or diaphragm '22 in which an apertured grid structure 23 is centrally mounted. An adjacent associated parallel grid structure 24 is mounted on the end of a pole 25 formed by a reentrant integral portion of the resonator bottom wall. Grids 23 and 24, which are thus flexibly interconnected, may be of any suitable form, such as wire mesh or the grids disclosed in United States Letters Patent No. 2,261,154.

Resonator 2| is preferably made of copper, with flexible wall 22 comprising an annularly crimped area which has sufficient resiliency to tend to return to its illustrated position where the general plane of wall 22 is normal to the resonator axis.

The bottom resonator wall and platform 15 are apertured for rigidly mounting the inner end of a coaxial conductor transmission line 26 having a conductive loop 21 suitably disposed within the resonator chamber. Line 26 extends in vacuum tight relation through base 12 so as to be accessible for energy extraction. Any desired number of such lines may be provided.

Platform I5 is centrally apertured so that a suitable cathode 28 upstanding from base IZ may to entrance grid 24 of the resonator. Cathode 28 is suitably energized by connection to selected prongs l3.

A metal collar 29, rigid with exit grid 23 and the adjacent portion of flexible wall 22, is formed with an enlarged flange 33 in which is seated a disc 3| of insulating material centrally supporting a metal block 32. Block 32 has its lower face formed as an arcuate reflector 33 in alignment with grids 23 and 24 and cathode 23. Reflector 33 is thus mounted at a fixed distance from exit grid 23. A lead I33 (not fully shown) for applying a desired electrical potential to reflector 33 is connected between a prong l3 and block 32.

A generally U-shaped rigid bracket 34 has its opposite legs secured to flange and also holds disc 3| in its seat.

An elongated relatively rigid strut or rod 35 has its lower end tightly secured to bracket 34, and its upper end fastened as by screw 35 to a metal block 31 seated on a disc 38 of insulating material fixedly mounted on plate I 8 which is suitably apertured for free passage of the strut. Strut 35 is preferably made of stainless steel or some material having a large coefficient of thermal expansion, and is preferably axial with the resonator. Strut 35 is thereby anchored and fixedly secured at opposite ends to grids 23 and 24, so as to determine their spacing. I A collar 39 of insulating material is suitably fastened upon the top of bracket 34 for supporting a cylindrical electron emission electrode structure 4| symmetrically disposed about strut 35. Emission electrode 4| comprises a heater coil 42 and an internal oxide coated electron emission surface 43. Coil 42 has one end electrically connected to the normally grounded frame, as by wire 44, and its other end connected by wire 45 to a suitable prong 53. For clarity of disclosure, all of wire 45 is not shown.

Collar 39 also supports a cylindrical wire mesh grid electrode 46 symmetrical with and between surface 43 and strut 35. A suitable potential is applied to grid 46 as by a lead 47 connected to an adjustable power source 48. Preferably grid 46 is maintained positive with respect to surface 43 as indicated, and adjustment of tap 49 changes the potential on grid 46. Lead 4'! and power source 43 have been illustrated diagrammatically in Figure 1 for clarity of disclosure. It will be understood, however, that in practice lead 41 will extend to a prong l3, which will make contact through a suitable socket with source 48.

Electrode 46 need not be a grid but may be any electron control electrode of suitable structure. For some purposes, grid 46 may be omitted and the supply of electrons impacting strut 35 controlled simply by varying the power supply to coil 42.

Where an indirectly heated emitter such as 4| is employed about strut 35, there may be an appreciable time lag in heating and cooling thereof. This time lag may be considerably reduced by using a simple filamentary emitter such as a directly heated tungsten wire coil filament arranged about strut 35, in place of emitter 4!. Another short time lag arrangement embodies a nickel wire spiral coil filament emitter arranged about strut 35 and coated directly with emission material.

In operation, an electron stream from cathode 28 is projected through grids 24, 23 and is returned into the resonator through exit grid 23 by reflector 33. According to accepted theories of operation of such devices, the electron stream excites and maintains an oscillatory electromagnetic field within resonator 2!. For detailed discussion of such theory Of operation based on velocity modulation of the electrons in the stream, reference is made to United States Letters Patent No. 2,- 250,511, as such details of operation do not comprise part of the present invention. It is important to the invention only to understand that resonator 2i contains an electromagnetic field or circuit regardless of how the latter is produced.

Closely spaced grids 23 and 24 bound field regions of maximum intensity. Hence, control over the grid spacing will afford optimum control over the frequency of the energy which may be extracted on line 26.

In this phase of the invention the grid spacing, and consequently the shape and volume of resonator 2?, is controllabiy varied by controllably varying the length of strut 35. Upon energization of electrode 4i, electrons emitted from surface 43 bombard strut 35, thereby heating strut 35 substantially in proportion to the power input to electrode 45. When heated, strut 35 expands in length as power is expended in heating it and, since its upper end is anchored on the frame, its downwardly displaced lower end acts through bracket 34 and collar 39 to displace grid 23 toward grid 2:3 as permitted by the flexibility of wall 22. While electrode M is shown as coextensive with only a portion of strut 35, it may be made coextensive with any length of strut 35 according to the heating effect desired.

Electron bombardment of strut 35 from surface 43 is accurately and sensitively controlled by varying the potential of grid 43 by adjustment of tap 49. As grid 43 is made more positive, more electrons are drawn from surface 43 and permitted to impact strut 35, to thereby increase the temperature and elongation of strut 35. As grid 46 is made less positive, fewer electrons from surface 43 are permitted to impact strut 3'5, thereby decreasing the temperature and length of strut 35.

Operation of the above-described arrangement for controlling the temperature of strut 35 may be compared to operation of the conventional three element vacuum tube, with strut 35 and electrode 4! corresponding to the usual anode and cathode, respectively, and grid 46 functioning as the usual control grid. The top of bracket 34 is designed to serve as a shield for preventing stray electromagnetic fields from the strut heating unit from affecting the operation of resonator 2!.

Thus, as strut 35 is elongated by an increase in temperature, grids 23 and 24 approach each other and the resonant frequency of resonator 2| is decreased. As strut 35 is shortened by any decrease in temperature, grids 23 and 24 are separated and the resonant frequency or" resonator 2i is increased. The space charge control afforded by grid 46 gives instantaneous and reliable control over the heating power for strut 35. This manner of frequency control is especially useful for automatic frequency regulation systems where instantaneously responsive control action is important.

While, in Figure l, I have illustrated the expansible and contractible frequency control memher as a rigid strut which effects positive frequency control motion in opposite directions, it will be apparent that this phase of the invention is also applicable to devices wherein strut 35 is replaced by an expansible and contractible nonrigid member such as a tungsten wire connected between block 31 and bracket 34 and biased by a suitable spring tending to keep the wire taut as shown in Fig. 1A.

Figures 2 and 3 illustrate further similar embodiments of the invention wherein the electron stream, after interaction with a hollow resonator device, is employed to actuate frequency control in the device.

Referring to Figure 2, spaced hollow cylindrical resonators 5i and 52 having adjacent walls rigid with a drift tube 53 are provided with flexible opposite end walls 54 and 55. Grids 56 and 51 are provided in wall 54 and the adjacent end of tube 53, and grids 58 and 59 are provided in wall 55 and the adjacent end of tube 53.

A collar 59, rigid with grid 55, supports a suit able cathode 52. The end of collar 5! is vacuum sealed by a glass or like cap 53 through which extend leads for energizin the cathode.

At the other end of the device, an elongated hollow strut or tube 55 of stainless steel or like high coefficient of thermal expansion material is rigid with exit grid 58 and has its outer end anchored in a suitable base or support 55. The whole hollow resonator device is further mounted on base 65 by rigid attachment to drift tube 55 as illustrated.

Tube 54 has securely fastened thereto a radially and longitudinally extending bar 55 pivotally connected to one end of a lever 5l which has its other end pivoted on collar 5|. Intermediate its ends, lever 5'! is fulcrumed upon a pivot point 58 rigid with drift tube 53 as illustrated.

A suitable electrode 69 which enters tube 54 through an aperture sealed by glass bead H is 1* energized from a suitable adjustable unidirectional power supply indicated at l2. Electrode 69 is maintained negative as will appear.

In operation, electrons in the electron stream from cathode 52 are velocity modulated by passage through resonator 5!, become Velocity grouped during passage along tube 58, and give up energy to resonator 52 before passing through exit grid 55. Operation of such a device as an amplifier or oscillator is explained in detail in United States Letters Patent No. 2,242,275.

The electron stream emergin from exit 58 represents considerable energy which is usually wasted by dissipation in cooled collector electrodes. In the present invention, negatively biased electrodes 59 repels, deflects and otherwise disperses the electrons in the emergent stream so that many of the electrons impact tube 65. Tube 54 is thus heated by electron bombardment, and the degree of such heating can be sensitively controlled by malzing electrode 59 more or less negative to control the electron dispersion.

Tube 54 elongates when its temperature is raised. Since the outer end of tube 54 is anchored with respect to grid 59, elongation of tube 54 causes grid 58 to become displaced toward grid 59 as permitted by flexible wall 55, to thereby decrease the frequency of resonator 52. Equivalent motion of grid 55 toward grid El to proportionately decrease the frequency of resonator 5B is obtained through the motion transmitting mechanism consisting of bar 55 and lever El.

Thus by controlling the power supplied to electrode 65, I may sensitively effect coordinated frequency control of gang tuning of both resonators 5! and 52.

Electrons which pass electrode 59 bombard the outer end of tube 65 thereby heating that end. This heat is, however, effectively dissipated due to the large surface for radiation at the tube end,

and due to conduction by the supportin base, and does not appreciably interfere with the above described frequency control.

In the embodiments of the invention of Figl and 2, I effect accurate and reliable frequency control with the expenditure of very little power. In Figure l, the power required to energize emitter ll is low, and the power required for control grid 15 is almost negligible. In Figure 2, the power required to actuate control electrode 59 is only of about the same magnitude as required for grid 55.

Since the movements necessary for frequency control are small, the inherent resilience of flexible wall 5 3, which is the same as wall 22, renders it unnecessary to employ lost motion linkage and keeps the mechanism tight under all operating conditions. Figure 3, however, illustrates a device of the type of Figure 2 where pivotal motion transmitting links are eliminated.

Resonator 52 is replaced by a resonator l3 having two flexible end walls l4 and 15 secured respectively to drift tube 55 and exit grid 58. Bar 55 and lever Bl are replaced by a rigid bar 16 having its opposite ends secured to drift tube 53 and substantially the medial portion of tube 64. Resonator 5! is replaced by a resonator ll which together with resonator l3 is rigidly mounted on base "it and has a flexible wall is connected to drift tube 55.

At its outer end, tube 64 is anchored to base '55, and bar l3 is secured to tube 64 about halfway between that fixed end and grid 58. This arrangement insures equal and corresponding displacement of each pair of grids when tube 54 changes in length. For example, when tube 64 increases its length upon being heated, grid 58 moves a predetermined distance toward grid 59 and displacement of bar 15 causes longitudinal 1 shift of tube 55 and both grids M and 59 about one-half that distance. Thus, the distance between each pair of grids 55, 51 and 58, 59 is decreased by one-half the total displacement of grid 58. The reverse operation is accomplished when tube 54 contracts.

The invention as applied to Figures 2 and 3 is equally applicable to amplifiers and oscillators where the two resonator chambers are usually of equal size, and to devices such as frequency multipliers including unequally sized resonators. In the latter case, the dimensions of lever 61 (Figure 2) and the point of attachment of bar 16 to tube 54 (Figure 3) may be selected to secure the required proportionate tunin displacement.

While I have shown control electrode 69 as within strut 55 in the path of the electron beam, it is within the scope of the invention to employ for the same purpose any electron beam deflectin electrode or member such, for example, as a suitably polarized magnet laterally outside tube 54.

Figure 4 illustrates a two hollow resonator device having individual thermally responsive frequency control members arranged for gang or differential tuning.

Two hollow cylindrical resonators 8| and 82 are connected by adjacent flexible walls 83 and 84, similar to wall 22, to a drift passage tube 85. Electron permeable grid pairs 8B, 8'! and B8, 89 are provided on opposite ends of tube and the adjacent resonator walls. A suitable cathode 9| is mounted in alignment with the grids, and the usual concentric line terminals are provided at 92 and 93. i

A heavy radial flange 94, rigid with drift tube 85, is disposed equally intermediate similar parallel flanges 95 and 96 rigid with grids 86 and 89 respectively. A plurality of circumferentially spaced adjustable screw assemblies such as indicated 97 and 98 are provided between the associated flanges as illustrated. Usually screw assemblies 97 and 98 are so adjusted as to produce some flexure of walls 83 and 84 so as to select a predetermined grid spacing and keep the parts tight.

One pair of the screw assemblies is thermally responsive and controllably energized as illustrated at the left of Figure 4. A strut 99 of stainless steel, aluminum, duralumin or the like is held between flange 94 and a short screw IOI threaded in flange 95. Similarly an aluminum or like strut I02 is held between flange 94 and a short screw I03 threaded in flange 96. Heater coils I04 and I05 of resistance Wire are electrically insulatingly mounted about rods 09 and I02 respectively.

A battery I06 has a resistor I07 in parallel therewith and both heater coils I04, I 05 are connected across the resistor so that adjustment of tap I08 similarly varies the power delivered to each coil. The lead to coil I04 also passes through an adjustable resistor I09 by which the relative power delivered to coils I04 and I 05 may be varied for trimming or differentially tuning the resonators.

Usually three 120 spaced tuning control assemblies are employed, two aligned screw assemblies 97, 98 and one aligned strut arrangement I04, I05. The small frequency control movements involved render any departures from parallelism of the grids due to such movement immaterial. If desired moreover, all three strut assemblies may be made identical with that at 304-, I05 and all connected to battery I05 and resistor I07 in the same manner. This will give accurate and exactly parallel relative displacement of the grids.

In operation, preliminary and rough frequency control adjustments are made with screw assemblies 91, 98 and screws II and I03. Expansion and contraction of rods 99 and I02 by controlled energization of coils I04, I05 result in corresponding displacements of grid pairs 86, 87 and 88, 89. If the inherent resiliency of walls 83 and 84 is not suflicient to keep the parts tight, tension springs (not shown) may be provided between the flanges.

Simultaneous and similar frequency control of each resonator BI and 82 can thus be obtained by adjustment of tap I08. This is electrical gang tuning. The frequency of resonator 8| may be independently varied by adjustment of resistor I09. This is a trimming adjustment to compensate for physical and/or electrical inequalities between the resonators.

I have found that, using struts 99, I02 of aluminum, an elongation of about 0.008 inch for a 300 C. rise in strut temperature. The amount of movement is ample for the usual required tuning range. If desired, struts 99 and I02 may be hollow with heater coils I04, I05 inside.

This gang tuning arrangement may be also accomplished by passing heating current directly through struts 99 and I02, the latter being suit ably insulated and rigidly connected to flanges 94, 95 and 96.

In Figure 5, a hollow resonator III has a rigid end wall I I2 and a flexible end wall I I3 cent-rally supporting a hollow post II4 which extends into the resonator to terminate adjacent wall H2. Suitable electron permeable grid structures H5 and II 6 are provided in wall I I2 and the inner end of post II 4. The outer end of post H4 is secured to a conventional pronged vacuum tube base I I7. On the other side of resonator III, a shallow cup-shaped metal reflector II 8 is suitably mounted within a glass or like sealing cap H9. Reflector IE8 is electrically connected to an external terminal I2I by which a suitable potential may be applied to the reflector. A cathode I22 extends within the post H4 in alignment with the grids and reflector.

Radial flanges I28 and I24 are provided rigid with grids II5 and H6 respectively. A plurality of circumferentiaily spaced adjustable length screw assemblies I25 (only one shown), each biased by associated tension springs I26, are provided between flanges I23 and I24, for manual frequency control adjustments.

Flanges I 23 and I24 are formed with aligned apertures in which hollow tubes I27 and I28 of Invar or some metal or alloy having a very low coeflicient of thermal expansion are fixed and extend in opposite directions. The end of tube I27 is closed by a plug I29 of insulating material in which is securely imbedded a conductive rod l3I, and the threaded outer end of rod I3! carries a reenforcing nut I32.

The outer end of tube I28 is closed by a suitable insulating plug I33 which is centrally apertured to slidingly receive a short conductive rod I34 having a threaded outer end I35 for mounting a knurled nut I35. A non-rigid expansible and contractible conductor I37, preferably a thin flexible tungsten strip or wire, is secured at opposite ends to rods I 3| and I34. Conductor I37 may be set at predetermined tension by adjustment of nut I36. A wire is preferable because it heats up and loses heat rapidly whereby lending itself to automatic tuning, and a tungsten wire is preferable because of its exceptional strength and toughness even at high heat. Any metal or alloy having a slow uniform expansion and contraction in response to heating by electrical current passing therethrough may be employed for wire I37. A suitable compression spring I38 is arranged between flanges I23 and I24 and surrounding conductor I37.

In operation, relative separation of grids I55 and I E and hence frequency control of resonator l I i may be effected by controllably energizing conduct-or I37 from an adjustable power source I30. As conductor I31 is increasingly heated, it expands in length, and spring I38 is permitted to increase the spacing between grids H5, IIG to thereby increase the resonator frequency. When conductor I 31 cools it contracts to produce the reverse frequency control. Springs 52% are not sufficiently strong to interfere with the above operation.

Besides permitting the use'of a long wire I37 for obtaining appreciable tuning motion, tubes I2? and I23 provide protective draft shields for wire I37.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the acompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. High frequency apparatus comprising a cavity resonator having a pair of aligned relatively movable electron-permeable wall portions, a rigid tuning strut coupled between said Wall portions and adapted upon expansion or contraction to alter the spacing of said wall portions and thereby to tune said resonator, said strut extending along the direction of alignment of said wall portions, a source of electrons adjacent to said strut and adapted to project electrons thereat to heat and alter the length of said strut, and a control electrode between said source and said strut for controlling the magnitude of the heating electron current from said source to said strut.

2. High frequency apparatus comprising a cavity resonator having a pair of relatively movable electron-permeable wall portions, a cathode on one side of said resonator, a reflector electrode on the other side of said resonator, a rigid tuning strut coupled between said wall portions and extending generally in alignment with said cathode, wall portions and reflector electrode and adapted upon expansion or contraction to alter the spacing of said wall portions and thereby to tune said resonator, a source of electrons adjacent to said strut and adapted to project electrons thereat to heat and alter the length of said strut, and a control electrode between said source and said strut for controlling the magnitude of the heating electron current from said source to said strut.

3. High frequency apparatus comprising a vacuum envelope, a cavity resonator within said envelope and having a pair of relatively movable electron-permeable electrodes, means in said envelope for projecting a stream of electrons through said electrodes to be velocity modulated by the field therebetween, means in said envelope for reflecting said electrons and reprojecting them through said electrodes to give up energy to said field, a. rigid elongated thermally expansible tuning strut in said envelope rigidly connected at each end to a respective one of said electrodes, said strut extending along the direction of said electron stream path, a cylindrical control grid surrounding said strut, and a cylindrical cathode surrounding said control grid, whereby said strut may be controllably heated by electrons from said cathode under the control of said control grid to adjust the spacing of said electrodes and thereby tune said resonator.

4. High frequency apparatus comprising a vacuum envelope, a cavity resonator within said envelope and having a pair of relatively movable electron-permeable electrodes, means in said envelope for projecting a stream of electrons through said electrodes, a rigid elongated thermally expansible tuning strut in said envelope rigidly connected at each end to a respective one of said electrodes, said strut extending substantially parallel to the direction of said electron stream path, a control grid surrounding said strut, and a cathode surrounding said control grid, whereby said strut may be controllably heated by electrons from said cathode under the control of said control grid to adjust the spacing of said electrodes and thereby tune said resonator.

5. High frequency apparatus comprising a cavity resonator having a pair of relatively movable electron-permeable electrodes, means in said apparatus for projecting a stream of electrons through said electrodes, a thermally expansible tuning member rigidly connected at each end to a respective one of said electrodes, said member extending substantially parallel to the direction of said electron stream path, a control grid adjacent said member, and a cathode aligned with said control grid and said member whereby said member may be controllably heated by electrons from said cathode under the control of said control grid to adjust the spacing of said electrodes and thereby tune said resonator.

SIGURD F. VARIAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,419,547 Ehret June 13, 1922 2,095,981 Hansell Oct. 19, 1937 2,167,201 Dallenbach July 25, 1939 2,183,215 Dow Dec. 12, 1939 2,216,170 George Oct. 1, 1940 2,250,511 Varian et a1 July 29, 1941 2,251,085 Unk July 29, 1941 2,259,690 Hansen et al. Oct. 21, 1941 2,284,405 McArthur May 26, 1942 2,338,237 Fremlin Jan. 4, 1944 2,414,785 Harrison et a1 Jan. 21, 1947 

